The present invention relates to a semiconductor device including an MISFET and a method for fabricating the same, and more particularly relates to a measure for increasing the mobility of carriers.
When a stress is generated in a semiconductor crystal layer, a crystal-lattice constant varies and a band structure is changed, so that the mobility of carriers is changed. This phenomenon has been known as the “piezo resistivity effect”. Whether the carrier mobility is increased or reduced differs depending on the plane direction of a substrate, the direction in which carriers move, and whether the stress is a tensile stress or a compressive stress. For example, in an Si (100) substrate, i.e., a silicon substrate of which the principal surface is the {100} plane, assume that carriers move in the [011] direction. When carriers are electrons, with a tensile stress generated in the direction in which electrons in a channel region move, the mobility of the carriers is increased. On the other hand, when carriers are holes, with a compressive stress generated in the direction in which holes in a channel region move, the mobility of the carriers is increased. The increase rate of carrier mobility is proportional to the size of a stress.
In this connection, conventionally, there have been proposals for increasing carrier mobility by applying a stress to a semiconductor crystal layer to increase the operation speed of transistors and the like. For example, in Reference 1, an entire semiconductor substrate is bent using an external device, thereby generating a stress in an active region of a transistor.